The various events and processes involved in a digital communication system can be organized or classified into several levels or layers, each layer being more abstract than the layer below. The OSI 7-layer system is the internationally accepted model for such systems; in the OSI model, the processes or events are classified into 7 layers, which can be summarized as follows:
1 Physical PA1 2 Data link PA1 3 Network PA1 4 Transport PA1 5 Session PA1 6 Presentation PA1 7 Application.
A major advantage of this classification into layers is that it enables the specification of protocols to be similarly divided up. In general, layer N-1 provides one or more communication services to layer N. That is, the service provided by layer N is defined in terms of a protocol which uses the services of layer N-1. Layer N in turn provides one or more communication services to layer N +1, and so on.
The Physical layer defines services applicable to a physical link between two (or more) intercommunicating units. The protocol supporting this service is typically defined in terms of electrical signals on wires or optical cables. The Datalink layer builds on the services of the physical layer to provide a service to the Network layer, and so on up through the layers. Each layer provides a different service; for example, the network layer includes protocols to determine the best route through a network, while the transport layer provides a common service over different networks layers.
The design of each layer is thus largely independent of the design of the other layers. In particular, a change in the implementation of a layer requires only relatively minor adjustments to the interface functions with the adjacent layers.